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Estimation and sample size calculations for correlated binary error
rates of biometric identification devices
Michael E. Schuckers,211 Valentine Hall, Department of Mathematics
Saint Lawrence University, Canton, NY 13617 and
Center for Identification Technology Research, West Virginia University
[email protected]
November 14, 2003
KEYWORDS: False Accept Rate, False
Reject Rate, Logit Transformation, Betabinomial, Intra-cluster Correlation
1
Background
Biometric Identification Devices (BID’s) compare
a physiological measurement of an individual to a
database of stored templates. The goal of any BID
is to correctly match those two quantities. When the
comparison between the measurement and the template is performed the result is either an ”accept” or a
”reject.” For a variety of reasons, errors occur in this
process. Consequently, false rejects and false accepts
are made. As acknowledged in a variety of papers,
e.g. Wayman (1999), Mansfield and Wayman (2002),
there is a need for assessing the uncertainty in these
error rates for a BID.
Despite the binary nature of the outcome from the
matching process, it is well known that a binomial
model is not appropriate for estimating false reject
rates (FRRs) and false accept rates (FARs). This is
also implicitly noted by Wayman (1999). Thus, other
methods that do not depend on the binomial distribution are needed. Some recent work has made headway
on this topic. In Bolle et al. (2000) the authors propose using resampling methods to approximate the
cumulative density function. This methodology, the
so-called ”subset bootstrap” or ”block bootstrap”,
has received some acceptance. Schuckers (2003) has
proposed use of the Beta-binomial distribution which
is a generalization of the binomial distribution. Like
the ”subset bootstrap”, this method assumes conditional independence of the responses for each individual. Utilizing the Beta-binomial, Schuckers outlines a
procedure for estimating an error rate when multiple
users attempt to match multiple times.
One of the primary motivations for assessing sampling variability in FARs and FRRs is the need to determine the sample size necessary to estimate a given
error rate to within a specified margin of error,e. g.
Snedecor and Cochran (1995). Sample size calculations exist for basic sampling distributions such as the
Gaussian and the binomial. Calculations of this kind
are generally simpler for parametric methods such as
the Beta-binomial than they are for non-parametric
methods such as the ”subset bootstrap.”
In this paper we present two methodologies for
confidence interval estimation and sample size calculations. We follow a general model formulated
by Moore (1987) for dealing with data that exhibits
larger variation than would be appropriate under a
binomial sampling distribution. In the next section
we introduce this model and the notation we will use
throughout this paper. Section 3 introduces confidence interval and sample size estimation based on
this model. There we present a simulation study to
assess the performance of this model. Section 4 proposes a second methodology for confidence interval
and sample size estimation. This one is based on a
log-odds, or logit, transformation of the error rate. by denoting them with a ”hat.” For example, π
ˆ will
We also present results from a simulation study for represent the estimate of π, the overall error rate.
this model. Finally, Section 5 is a discussion of the
results presented here and their implications.
3
2
Extravariation Model
Several approaches to modelling the error rates from
a BID have been developed. Here we take a parametric approach. Schuckers (2003) presented an extravariation model for estimating FARs and FRRs.
That approach was based on the Beta-binomial distribution which assumes that error rates for each individual follow a Beta distribution. See Schuckers
(2003) for further details. Here we follow Moore
(1987) in assuming the first two moments of that
model but not in the form of the distribution; that
is, we no longer use the assumptions of the Beta distribution. We give details of this model below.
In order to further understand this model, it is necessary to define some notation. We assume an underlying error rate, either FAR or FRR, of π. Following
Mansfield and Wayman (2002), let n be the number
of individuals tested and let mi be the number of
times that the ith individual is tested, i = 1, 2, . . . , n.
For simplicity we will assume that mi = m, for any i.
All of the methods given below have generalizations
that allow for different numbers of tests per individual. Then for the ith individual, let Xi represent the
observed number of errors from the m attempts and
let pi represent the percentage of errors from the m
observed attempts, pi = Xi /m.
We assume here that:
E[Xi ]
V ar[Xi ]
= mπ
= mπ(1 − π)(1 + (m − 1)ρ)
(1)
where ρ is a term representing the degree of extravariation in the model. ρ is often referred to as the
intra-class correlation coefficient, e. g. (Snedecor and
Cochran, 1995). In Appendix A, we give examples of
data with identical values of π
ˆ and two different values of ρ to illustrate how ρ influences the data that is
observed. For the rest of this paper we will use traditional statistical notation for estimates of parameters
Traditional confidence intervals
In the previous section, we introduced an notation for
the extravariation model. Here we will use this model
for estimating π. Suppose that we have the observed
Xi ’s from a test of a biometric identification device.
We can then use that data to estimate the parameters of our model. Here we will focus on two aspects
of inference: confidence intervals and sample size calculations. Specifically, we derive confidence intervals
for the error rate π and the sample size needed to
achieve a specified level of confidence. Let
P
Xi
π
ˆ = P
mi
BM S − W M S
ρˆ =
BM S + (m0 − 1)W M S
where
P
mi (pi − π
ˆ )2
BM S =
,
P n−1
mi pi (1 − pi )
WMS =
n(m0 − 1)
and
P
2
(mi − m)
m0 = m −
mn(n − 1)
BMS and WMS represent between mean squares and
within mean squares respectively. This estimation
procedure for ρ is given by Lui et al. (1996).
Thus we have an estimate, π
ˆ and we can evaluate it’s standard error following equation 1 assuming
that the individuals tested are conditionally independent of each other. That is, we assume the Xi ’s are
conditionally independent. The standard error of π
ˆ
is then
P
Xi
ˆ
ˆ
V [ˆ
π] = V [
]
mn X
= (mn)−2
Vˆ [Xi ]
=
π
ˆ (1 − π
ˆ )(1 + (m − 1)ˆ
ρ)
.
mn
(2)
(Note that in Schuckers (2003) they refer to a quanTable 1: Empirical Coverage Probabilities for Confitity C which here is (1 + (m − 1)ρ).) Now we can
dence Interval, n = 1000
create a nominally 100 × (1 − α)% confidence interval
for π from this. Assuming a Gaussian distribution for
m=5
the sampling distribution of π
ˆ , we get the following
0.01
0.1
0.4
π\ρ 0.001
interval
0.002 0.924 0.921 0.891 0.862
1/2
π
ˆ (1 − π
ˆ )(1 + (m − 1)ˆ
ρ)
0.004 0.943 0.943 0.931 0.914
(3)
π
ˆ ± z1− α2
mn
0.008 0.940 0.947 0.951 0.931
0.010 0.952 0.954 0.947 0.928
α
th
percentile of a
where z α represents the 1 −
1− 2
2
Gaussian distribution. Here we assume Normality
of the sampling distribution of π
ˆ is assumed. That
assumption relies on the asymptotic properties of
these estimates, (Moore, 1986). To test the appropriateness of this we simulated data from a variety of
different scenarios. Under each scenario, 1000 data
sets were generated and from each data set a 95%
confidence interval was calculated. The percentage
of times that π is captured in these intervals is investigated and referred to as coverage. For a 95%
confidence interval, we should expect the coverage
to be 95%. We consider full factorial simulation
study using the following value: n = (1000, 2000),
m = (5, 10), π = (0.002, 0.004, 0.008, 0.010), and
ρ = (0.001, 0.01, 0.1, 0.4). These values were chosen to determine their impact on the coverage of the
confidence intervals. Specifically, these values of π
were chose to be representative of possible values for
a BID, while the chosen values of ρ were chosen to
represent a larger range than would be expected. Performance for all of the methods given in this paper is
exemplary when π is between 0.1 and 0.9. Thus we
investigate how well these methods do when those
conditions are not met.
To generate data from each of these scenarios, we
follow Oman and Zucker (2001) who recently presented a methodology for generating correlated binary data with the specific marginal characteristics
that we have here. Thus this simulation makes no
assumptions about the form of the data other than
that each observation is generated to have probability of error π and intra-class correlation ρ. Results
from these simulations can be found for n = 1000 and
n = 2000 in Table 1 and Table 2, respectively.
Before summarizing the results from these tables,
it is appropriate to make some remarks concerning
π\ρ
0.002
0.004
0.008
0.010
0.001
0.944
0.942
0.947
0.953
m = 10
0.01
0.950
0.948
0.941
0.966
0.1
0.908
0.916
0.941
0.937
0.4
0.855
0.904
0.928
0.936
Each line represents 1000 simulated data sets.
interpretation of these results. First, since we are
dealing with simulations, we should pay attention to
overall trends rather than to specific outcomes. If we
ran these same simulations again, we would see slight
changes in the coverages of individual scenarios but
the overall trends should remain. Second, this way
of evaluating how well a methodology, in this case a
confidence interval, does is appropriate. Though it
may seem artificial, using simulated data, is the best
way to evaluate an applied confidence interval, since
one can know how often the parameter, in this case
π, is captured by the confidence interval. Observed
test data does not give an accurate guide to how well
a method performs since there is no way to know the
actual error rate, π.
Turning our attention to the results in Table 1 and
Table 2, several clear patterns emerge. Coverage increases as π increases, as ρ decreases, as n increases
and as m increases. This is exactly as we would have
expected. More observations should increase our ability to accurately estimate π. Similarly the assumption of Normality will be most appropriate when π
is moderate (far from zero and far from one) and
Table 2: Empirical Coverage Probabilities for Confidence Interval, n = 2000
π\ρ
0.002
0.004
0.008
0.010
0.001
0.940
0.954
0.957
0.950
m=5
0.01
0.943
0.945
0.942
0.948
0.1
0.929
0.932
0.942
0.927
0.4
0.904
0.919
0.936
0.924
π\ρ
0.002
0.004
0.008
0.010
0.001
0.947
0.946
0.933
0.958
m = 10
0.01
0.927
0.950
0.957
0.946
0.1
0.928
0.927
0.931
0.927
0.4
0.889
0.914
0.915
0.945
Each line represents 1000 simulated data sets.
when ρ is small. The confidence interval performs
well when π > 0.002 and ρ < 0.1. There is quite a
range of coverages from a high of 0.966 to a low of
0.855. One way to think about ρ is that it governs
how much ’independent’ observations can be found
in the data. Higher values of ρ indicate that there
is less ’independent’ information in the data. This
is not surprising since data of this kind will be difficult to assess because of the high degree of correlation within an individual. This methodology would
be acceptable for very large samples when the error
rate, π is far from zero. However, this is often not
the case for BID’s. The difficulty with inference using the above methodology is that the assumption of
Normality for the sample distribution of π
ˆ is only approximate when π is moderate or when ρ is large or
both. Below we present a remedy for this.
4
Transformed Confidence Intervals
As evidenced above one of the traditional difficulties with estimation of proportions near zero or one
is that sampling distribution are only asymptotically
Normal. One statistical method that has been used
to compensate for this is to transform the calculations
to another scale. Many transformations for proportions have been proposed including the logit, probit
and arcsin of the square root. See Agresti (1990)
for full details of these methods. Below we use the
logit or log-odds transformation to create confidence
intervals for the error rate π.
4.1
Logit function
The logit or log-odds transformation is one of the
most commonly used transformations in Statistics.
π
). This is the logarithm of
Define logit(π) = log( 1−π
the odds of an error occurring. The logit function
has a domain of (0, 1) and a range of (−∞, ∞). One
advantage of using the logit transformation is that
we move from a bounded parameter space to on unbounded one. Thus, the possibility exists that the
transformed sampling distribution should be closer
to Normality than the untransformed one.
4.2
Confidence intervals
Our approach here is this. We transform our estimand π, create a confidence interval for the transformed quantity, then invert the transformation back
to the original scale. We do this in the following
eγ
manner. Let γˆ = logit(ˆ
π ), ilogit(γ) = 1+e
γ and note
the ilogit is the inverse of the logit function. Thus
we can create a 100(1 − α)% confidence interval using γˆ . To do this we use a Delta method expansion
for estimated the standard error of γˆ . (The Delta
method, as it is known in the statistical literature, is
simply a one step Taylor series expansion. See Agresti
(1990) for details.) Then our confidence interval on
the transformed scale is
γˆ ± z
1− α
2
1 + (m − 1)ˆ
ρ
π
ˆ (1 − π
ˆ )mn
12
(4)
Table 3: Empirical Coverage Probabilities for Logit
Confidence Interval, n = 1000
Table 4: Empirical Coverage Probabilities for Logit
Confidence Interval, n = 2000
π\ρ
0.002
0.004
0.008
0.010
0.001
0.964
0.961
0.951
0.958
m=5
0.01
0.951
0.952
0.960
0.959
0.1
0.964
0.945
0.955
0.952
0.4
0.907
0.926
0.954
0.954
π\ρ
0.002
0.004
0.008
0.010
0.001
0.924
0.943
0.940
0.952
m=5
0.01
0.921
0.943
0.947
0.954
0.1
0.891
0.931
0.951
0.947
0.4
0.862
0.914
0.931
0.928
π\ρ
0.002
0.004
0.008
0.010
0.001
0.941
0.942
0.947
0.955
m = 10
0.01
0.930
0.933
0.963
0.963
0.1
0.929
0.942
0.928
0.944
0.4
0.839
0.923
0.939
0.941
π\ρ
0.002
0.004
0.008
0.010
0.001
0.944
0.942
0.947
0.953
m = 10
0.01
0.950
0.948
0.941
0.966
0.1
0.908
0.916
0.941
0.937
0.4
0.855
0.904
0.928
0.936
Each line represents 1000 simulated data sets.
Each line represents 1000 simulated data sets.
We will refer to the endpoints of this interval as L
and U for lower and upper respectively. The final
step for making a confidence interval for π is to take
the ilogit of both endpoints of this interval. Hence
we get (ilogit(L),ilogit(U)).
The interval (ilogit(L),ilogit(U)) is asymmetric because the logit is not a linear transformation. Thus
we do not have a traditional confidence interval that
is plus or minus a margin of error. However, this
interval has the same properties as other confidence
intervals. To assess how well this interval performs
we repeated the simulation done in section 3. Output from these simulations are summarized in Table
3 and Table 4 which contain output when n = 1000
and when n = 2000 respectively. Again coverage
should be close to 95% for a 95% confidence interval. Looking at the results found in Tables 3 and 4,
we note that there are very similar patterns to those
found in the previous section. As before we are interested in trends rather than specific values. In general, coverage increases as π increases, as ρ decreases,
as m increases and as n increases. Coverages range
from a high of 0.965 to a low of 0.907. Coverage was
only poor when π = 0.002 and ρ = 0.4. Otherwise
the confidence interval based on a logit transformation performed quite well. Overall, coverage for the
logit confidence interval (LCI) is higher than for the
untransformed confidence interval (UCI). One possible remedy for increasing coverage slightly, when
ρ > 0.1, would be to use a percentile from the Students t-distribution rather than a percentile from the
Gaussian distribution.
4.3
Sample size and power calculations
One important statistical tool that is not found in
the BID literature is sample size calculations. This
is an extremely important tool for testing of a BID.
Since the transformation to the logit scale give approximately correct coverage, we can use the confidence interval given in Equation 4 to determine approximate sample size calculations. The asymmetry
of the logit interval provides a challenge relative to
the typical sample size calculation. Here rather than
specifying the margin of error as is typical, for exam-
ple see Snedecor and Cochran (1995), we will specify
the maximum value for the confidence interval. Given
the nature of BID’s where lower error rates are better, it seems somewhat natural to specify the highest
acceptable value for the range of the interval.
Given equation 4, we can determine the appropriate sample size needed to estimate π with a certain
level of confidence, 1 − α, to be a specified maximum
value, πmax . The difficulty with calculating a sample
size is that there are actually two sample size components to BID testing: the number of individuals and
the number of times each individual is tested. This
makes a single universal formula for the sample size
intractable. However, we can provide a conditional
solution. We follow the following steps. First, specify appropriate values for π, ρ, πmax , and 1 − α. As
with any sample size calculations these values can be
based on prior knowledge, previous studies or simply
guesses. Second, fix m, the number of attempts per
person. Third solve for n, the number of individuals
to be tested. Find n via the following equation, given
the other quantities:
are crucial to this assessment. Because of variability
in any estimate, it is important to be able to create inferential intervals for these error rates. In this
paper we have presented two methodologies for creating a confidence interval for an error rate. The
LCI, Equation 4 above, had superior performance for
the simulation study described here. Though that
study presented results only for 95% confidence intervals, it is reasonable to assume performance will
be similar for other confidence levels. One possible
improvement of coverage would be to use the appropriate percentile from a Student’s t-distribution with
m degrees of freedom rather than from the Gaussian
distribution. For example, if we were interested in
making a 95% confidence interval when m = 5, then
we would use t = 4.773 in place of the z in Equations 4 and 5. Another possibility would be to use
a simple function of m such as 2m. More research
is needed to understand this topic fully. This value
would be conservative in that it should give coverage
(and hence confidence intervals) that is larger than
necessary. (An initial simulation of 1000 data sets
using this methodology when π = 0.002, ρ = 0.4,
!2
2
z1−
α
n = 1000, and m = 5 gives coverage of 0.994.)
mπ(1
−
π)
2
.
n=
The most immediate consequence of the perforlogit(πmax ) − logit(π)
(1 + (m − 1)ρ)
mance of the LCI is that we can ’invert’ that calcu(5)
lation to get sample size calculations. Because of the
The above follows directly from Equation 4. For exasymmetry of this confidence interval, it is necessary
ample, suppose that you wanted to estimate π to a
to estimate the parameters of the model, π and ρ, as
max of = 0.01 with 99% confidence. You estimate
well as to determine the upper bound for the confiπ to be 0.005 and ρ to be 0.01. If we plan on testing
dence interval. As outlined above, this now gives BID
each individual 5 times then we would need to test,
testers an important tool for determining the numthen
ber of individuals and the number of attempts per
2
2.576
5(0.005)(0.995) individual necessary to create a confidence interval.
n =
logit(0.01) − logit(0.005)
(1 + (5 − 1)0.01)Further work is necessary here to develop methodology that will provide power calculations and further
= 569.14.
(6)
refine this work.
In this paper we have provided simple, efficient apSo we would need to test 570 individuals 5 times each
proximations
based on appropriate models that perto achieve the specified level of confidence.
form quite well. These are valuable initial steps. Further work is necessary to be able to compare the performance of this methodology to methodologies al5 Discussion
ready in use. In addition, it is important to advance
For any BID, its matching performance is often criti- our understanding of how this model performs.
Acknowlegements: This research was made poscal to the success of the product from the viewpoint.
The error rates, the false accept and the false reject, sible by generous funding from the Center for Iden-
tification Technology Research (CITeR) at the West Wayman, J. L. (1999). Biometrics:Personal IdentiVirginia University. The author would also like to
fication in Networked Society, chapter 17. Kluwer
thank Robin Lock, Stephanie Caswell Schuckers, Anil
Academic Publishers, Boston.
Jain, Larry Hornak, Jeff Dunn and Bojan Cukic for
insightful discussions regarding this paper.
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